1. Technical Field
The present invention relates to an optical information detecting method, an optical information detector, and a data sampling method, and more particularly, to an optical information detecting method, an optical information detector, a data sampling method, in which optical information can be efficiently detected from an optical information recording medium.
2. Related Art
Recently, with increase in requirement for a next-generation storage system having large storage capacity, an optical information processing system using holography, that is, a holographic optical information processing system, has attracted attention.
In the holographic optical information processing system, by irradiating a signal beam containing data and a reference beam at an angle different from that of the signal beam to a predetermined position of an optical information recording medium and intersecting two beams each other, an interference pattern is recorded in the optical information recording medium. At the time of reproducing the stored information, using diffracted beam generated from the interference pattern by irradiating the reference beam to the stored interference pattern, original data are reproduced.
In the holographic optical information processing system, data can be superposed and stored at the same position of the optical information recording medium by the use of a variety of multiplexing methods and the superposed and stored data can be separated and reproduced. Accordingly, it is possible to embody a data storage system with a super large capacity. Examples of the multiplexing methods can include an angular multiplexing method, a wavelength multiplexing method, and a phase code multiplexing method.
On the other hand, in the holographic optical information processing system, digital data are processed in units of predetermined pages and a page which is unit data is called a data page. That is, the holographic optical information processing system processes data in units of data pages. The optical information processing operation in units of data pages is described in detail in U.S. Pat. No. 5,838,650 and Japanese Unexamined Patent Publication No. 1998-97792.
For example, in the holographic optical information processing system, input data are encoded in units of data pages, the encoded binary data are allowed to correspond to pixels to create two-dimensional images of data pages, the two-dimensional images of data pages are loaded to a signal beam, and the signal beam is irradiated to the optical information medium. This optical modulation is carried out by a spatial light modulator (SLM).
At this time, a reference beam is irradiated to the optical information recording medium at an angle different from the irradiation angle of the signal beam. The signal beam and the reference beam interfere with each other in the optical information recording medium and the images of the data pages loaded to the signal beams are recorded in the form of interference patterns in the optical information recording medium.
The images of the data pages recorded in the optical information recording medium can be reproduced by irradiating the reference beam to the interference patterns. The reproduced images of the data pages can be detected by a light receiving device such as a complementary metal-oxide semiconductor (CMOS) or a charge coupled device (CCD). The detected images of the data pages are reproduced in original data through predetermined signal processing and decoding operations.
On the other hand, a variety of sampling methods as follows can be used for detecting the images of the data pages by the use of the light receiving device.
1. 1:1 Pixel Matching Method
A 1:1 pixel matching method is a method of matching pixels of the light receiving device (hereinafter, referred to as “light receiving pixels”) with pixels of the reproduced image of the data page (hereinafter, referred to as “data pixels”) in 1:1. In the 1:1 pixel matching method, since one data pixel corresponds to one light receiving pixel, the storage density is high at the time of detecting the image.
At the time of actually reproducing the image of the data page, the position of the reproduced image formed on the light receiving device varies due to shrinkage or rotation of the optical information recording medium, and misalignment is caused. Accordingly, the data pixels and the detection pixels are not matched with each other.
However, in the 1:1 pixel matching method, when two kinds of pixels depart from each other by a half or more size of the data pixel, the image of the data page detected by the light receiving device can be severely degraded. When the departure of pixels is severe, it is not possible to obtain accurate information.
2. 1:3 Over-Sampling Method
A 1:3 over-sampling method is a method of detecting one data pixel by the use of 9 detection pixels (3×3). In the 1:3 over-sampling method, even when the departure between the data pixels and the detection pixels occurs, the detection pixel positioned at the center of the 9 detection pixels can detect the beam from the data pixel. Accordingly, everywhere the reproduced image of the data page is located in the light receiving device, it is possible to obtain data with high reliability from the image detected by the center detection pixel.
However, in the 1:3 over-sampling method, since 9 detection pixels are required for detecting one data pixel at the time of detecting the image, the storage density is too low. For example, when a light receiving device having 1200×1200 detection pixels is used, one data page can contain 400×400 pixels of data. Accordingly, stability of a system can be secured, but the storage capacity which is the best advantage of a holographic memory is degraded.
3. 1:2 Over-Sampling Method
A 1:2 over-sampling method is a method of detecting one data pixel by the use of 4 detection pixels (2×2). In the 1:2 over-sampling method, similarly to the 1:3 over-sampling method, even when a departure occurs between the data pixels and the detection pixels, one detection pixel of the 4 detection pixels can detect the beam from the data pixel. Accordingly, it is possible to obtain data with high reliability. However, in comparison with the pixel matching method, the 1:2 over-sampling method has a disadvantage that the storage density is 25% of that of the pixel matching method.
The known pixel matching method has an advantage of a large storage density but has a disadvantage of misalignment between pixels. The known 1:3 over-sampling method and 1:2 over-sampling method have an advantage of high reliability in detecting data but have a disadvantage of too small storage density. Therefore, an optical information detecting method capable of securing reliability in data detection and satisfying a high storage density has been required.